Optical glass

ABSTRACT

An optical glass having high-refractivity and low-dispersion properties and containing, by mol %, 0.1 to 40% of SiO 2 , 10 to 50% of B 2 O 3 , wherein the mass ratio of the content of SiO 2  to the content of B 2 O 3 , SiO 2 /B 2 O 3 , is 1 or less, 0.5 to 22% of ZnO, 5 to 50% of La 2 O 3 , and optionally other ingredients. The optical glass has a refractive index nd of 1.86 to 1.95 and an Abbe&#39;s number vd of (2.36−nd)/0.014 or more but less than 38, and a glass transition temperature of equal to or greater than 640° C.

This application is a divisional of application Ser. No. 12/865,574filed Sep. 8, 2010, now allowed, which in turn is the U.S. nationalphase of International Application No. PCT/JP2009/051402, filed 22 Jan.2009, which designated the U.S. and claims priority to JapaneseApplication No. 2008-019422, filed 30 Jan. 2008, the entire contents ofeach of which are hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to an optical glass in a broad sense, andspecifically, it relates to an optical glass having high-refractivitylow-dispersion properties, to a press-molding glass gob, an opticalelement formed of the above optical glass and its production process andto a process for the production of an optical element blank.

BACKGROUND ART

A lens formed of a high-refractivity low-dispersion glass enables thedownsizing of an optical system while correcting a chromatic aberrationwhen combined with a lens formed of a high-refractivity high-dispersionglass. It hence occupies an important place as an optical element forconstituting an image-sensing system or a projection optical system suchas a projector.

JP 2007-269584A discloses such a high-refractivity low-dispersion glass.The glass disclosed in JP 2007-269584A has a refractive index nd of 1.75to 2.00 and has a Ta₂O₅ content in the range of 0 to 25 mass %, and allof the glasses that have a refractive index nd of at least 1.85 containa large amount of Ta₂O₅. That is because the introduction of a largeamount of Ta₂O₅ is indispensable for securing glass stability in theregion of high refractivity such as a refractive index nd of 1.75 ormore. For such a high-refractivity low-dispersion glass, Ta₂O₅ is a mainand essential component.

Meanwhile, tantalum (Ta) is an element having a high rarity value and isin itself a very expensive substance. Moreover, rare metal prices arerecently soaring worldwide, and the supply of tantalum is deficient. Inthe field of glass production, tantalum as a raw material is deficient,and if such a situation continues, it may be no longer possible tomaintain the stable supply of high-refractivity low-dispersion glassesthat are essential and indispensable in the industry of opticalapparatuses.

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

Under the circumstances, it is an object of this invention to provide ahigh-refractivity low-dispersion optical glass that can be stablysupplied and has excellent glass stability, a press-molding glass goband an optical element that are formed of the above glass, and processesfor the production of an optical element blank and an optical element.

Means to Solve the Problems

For achieving the above object, the present inventors have made diligentstudies and as a result have found that the above object can be achievedby an optical glass having a specific glass composition, a specificrefractive index and a specific Abbe's number. This invention has beenaccordingly completed on the basis of the above finding.

That is, this invention provides

(1) an optical glass comprising, by mol %,

0.1 to 40% of SiO₂,

10 to 50% of B₂O₃,

0 to 10% of total of Li₂O, Na₂O and K₂O,

0 to 10% of total of MgO, CaO, SrO and BaO,

0.5 to 22% of ZnO,

5 to 50% of La₂O₃,

0.1 to 25% of Gd₂O₃,

0.1 to 20% of Y₂O₃,

0 to 20% of Yb₂O₃,

0 to 25% of ZrO₂,

0 to 25% of TiO₂,

0 to 20% of Nb₂O₅,

0 to 10% of Ta₂O₅,

over 0.1% but not more than 20% of WO₃,

0 to less than 3% of GeO₂,

0 to 10% of Bi₂O₃, and

0 to 10% of Al₂O₃,

the mass ratio of the content of SiO₂ to the content of B₂O₃, SiO₂/B₂O₃,being 1 or less,

the optical glass having a refractive index nd of 1.86 to 1.95 and anAbbe's number vd of (2.36−nd)/0.014 or more but less than 38,

(2) an optical glass as recited in the above (1), wherein the content ofTa₂O₅ is 0 to 7 mol %,

(3) an optical glass as recited in the above (1) or (2), which is aGe-free glass,

(4) a press-molding glass gob formed of the optical glass recited in anyone of the above (1) to (3),

(5) an optical element formed of the optical glass formed of the opticalglass recited in any one of the above (1) to (4),

(6) a process for the production of an optical element blank that iscompleted into an optical element by grinding and polishing,

the process comprising softening the press-molding glass gob recited inthe above (4) under heat and press-molding it,

(7) a process for the production of an optical element blank that iscompleted into an optical element by grinding and polishing,

the process comprising melting glass raw materials and press-molding theresultant molten glass to produce the optical element blank formed ofthe optical glass recited in any one of the above (1) to (3), and

(8) a process for the production of an optical element, which comprisescutting and polishing the optical element blank recited in the above (6)or (7).

Effect of the Invention

According to this invention, there can be provided a high-refractivitylow-dispersion optical glass that can be stably supplied and hasexcellent glass stability, a press-molding glass gob and an opticalelement that are formed of the above optical glass, and processes forthe production of an optical element blank and an optical element.

BRIEF EXPLANATION OF DRAWINGS

FIG. 1 is a photograph of devitrified glasses obtained in ComparativeExamples 1 and 2.

WORKING EMBODIMENTS OF THE INVENTION [Optical Glass]

First, the optical glass of this invention will be explained.

In the optical glass of this invention, the introduction of Ta₂O₅ thatis particularly expensive among glass components is decreased orlimited. Under this limitation, even when attempts are made to impart aglass with high-refractivity low-dispersion properties with maintainingdevitrification resistance, glass raw materials cannot form any glass ora formed glass is devitrified during a production process and is nolonger useable when the amount of Ta₂O₅ is simply decreased. Fordecreasing the introduction amount of Ta₂O₅ while avoiding theseproblems, the apportionment of components for imparting a highrefractive index is important.

In this invention, not only B₂O₃ and SiO₂ are introduced as componentsfor forming a glass network, but also La₂O₃, Gd₂O₃, Y₂O₃, WO₃ and ZnOthat are components for imparting a high refractive index are made to beco-present. In this invention, ZnO is an essential component not onlyfor improving meltability and decreasing a glass transition temperaturebut also for serving to attain high-refractivity low-dispersion andimprove devitrification resistance.

Under these conditions, the content of B₂O₃ and the content of SiO₂ arewell balanced to improve devitrification resistance, meltability andformability of a molten glass, and also well balanced with othercomponents, whereby the above object of this invention can be achieved.

The optical glass of this invention comprises, by mol %,

0.1 to 40% of SiO₂,

10 to 50% of B₂O₃,

0 to 10% of total of Li₂O, Na₂O and K₂O,

0 to 10% of total of MgO, CaO, SrO and BaO,

0.5 to 22% of ZnO,

5 to 50% of La₂O₃,

0.1 to 25% of Gd₂O₃,

0.1 to 20% of Y₂O₃,

0 to 20% of Yb₂O₃,

0 to 25% of ZrO₂,

0 to 25% of TiO₂,

0 to 20% of Nb₂O₅,

0 to 10% of Ta₂O₅,

over 0.1% but not more than 20% of WO₃,

0 to less than 3% of GeO₂,

0 to 10% of Bi₂O₃, and

0 to 10% of Al₂O₃,

the mass ratio of the content of SiO₂ to the content of B₂O₃, SiO₂/B₂O₃,being 1 or less,

the optical glass having a refractive index nd of 1.86 to 1.95 and anAbbe's number vd of (2.36−nd)/0.014 or more but less than 38.

(Reason for Limitation to Compositional Range)

Reasons for limitations to the above compositional ranges will beexplained below, while contents or total contents by % hereinafter standfor contents or total contents by mol % unless otherwise specified.

SiO₂ is a network-forming oxide and is an essential component necessaryfor maintaining glass stability and maintaining a viscosity suitable forforming a molten glass. When the content thereof is less than 0.1%, theglass stability decreases, and the viscosity of the glass during theformation of a molten glass decreases, so that the formability of aglass is degraded. Further, the glass is degraded in chemicaldurability. When the above content exceeds 40%, it is difficult torealize the desired refractive index, and further, the liquidustemperature and the glass transition temperature are increased. Further,there are caused problems that it is difficult to realize the desiredAbbe's number, that the meltability of the glass is degraded and thedevitrification resistance is degraded. The content of SiO₂ is thereforeadjusted to 0.1 to 40%. The content of SiO₂ is preferably in the rangeof 3 to 35%, more preferably in the range of 5 to 30%, still morepreferably in the range of 5 to 25%, yet more preferably in the range of7 to 22%, further more preferably in the range of 10 to 20%.

B₂O₃ is a network-forming component and is an essential componenteffective for maintaining the meltability of the glass and decreasingthe liquidus temperature. Further, it is also an effective component forimparting a low-dispersion property. When the content thereof is lessthan 10%, the glass stability decreases. When it exceeds 50%, not onlyit is difficult to satisfy the desired refractive index, but also thechemical durability is degraded. The content of B₂O₃ is thereforeadjusted to 10 to 50%. The content of B₂O₃ is preferably in the range of12 to 45%, more preferably in the range of 15 to 43%, still morepreferably in the range of 17 to 40%, yet more preferably in the rangeof 17 to 38%, further more preferably in the range of 18 to 35%.

For decreasing the liquidus temperature and improving thedevitrification resistance and the meltability, and for maintainingviscosity suitable for glass formability, the mass ratio of the contentof SiO₂ to the content of B₂O₃, SiO₂/B₂O₃, is adjusted to 1 or less. Theabove mass ratio refers to a value obtained by dividing the content ofSiO₂ by mass % with the content of B₂O₃ by mass %. When the above ratioexceeds 1, not only the liquidus temperature increases, but also thedevitrification resistance is degraded. And, the meltability is alsodegraded, and it is also difficult to realize the desired Abbe's number.

The above mass ratio is preferably in the range of 0.95 or less, morepreferably in the range of 0.90 or less.

Li₂O, Na₂O and K₂O are optional components that work to improve themeltability and decrease the glass transition temperature. When thetotal content of Li₂O, Na₂O and K₂O exceeds 10%, it is difficult torealize the desired refractive index, and the chemical durability isalso decreased. The total content of Li₂O, Na₂O and K₂O is thereforeadjusted to 0 to 10%. The total content of Li₂O, Na₂O and K₂O ispreferably in the range of 0 to 8%, more preferably in the range of 0 to6%, still more preferably 0 to 4%, yet more preferably in the range of 0to 2%, and it is further more preferred to incorporate none of the abovealkali metal oxides.

MgO, CaO, SrO and BaO work to improve the meltability of the glass andimprove the glass in light transmittance in a visible light region. Whenintroduced in the form of carbonates or nitrates, they also produce ananti-foaming effect. However, when the total content of these exceeds10%, the liquidus temperature increases and the devitrificationresistance is degraded. Besides these, the refractive index isdecreased, and the chemical durability is degraded. Therefore, the totalcontent of MgO, CaO, SrO and BaO is adjusted to 0 to 10%. The totalcontent of MgO, CaO, SrO and BaO is preferably in the range of 0 to 8%,more preferably in the range of 0 to 6%, still more preferably in therange of 0 to 4%, yet more preferably in the range of 0 to 2%. It isfurther more preferred to incorporate none of these alkaline earth metaloxides.

ZnO is an essential component useful for realizing the high-refractivitylow-dispersion properties, and it works to improve the glass inmeltability and devitrification resistance and decrease the liquidustemperature and the glass transition temperature. When the contentthereof is less than 0.5%, the refractive index may decrease, theliquidus temperature may increase or the devitrification resistance maybe degraded. On the other hand, when the above content exceeds 22%, itis difficult to realize the desired refractive index. Therefore, thecontent of ZnO is adjusted to 0.5 to 22%. The content of ZnO is morepreferably in the range of 0.5 to 20%, still more preferably in therange of 1 to 18%, yet more preferably in the range of 2 to 17%, furthermore preferably in the range of 3 to 17%, still further more preferablyin the range of 4 to 17%.

La₂O₃ is essential for realizing the high-refractivity low-dispersionproperties, and it also works to improve the chemical durability. Whenthe content thereof is less than 5%, it is difficult to obtain thedesired refractive index. When it exceeds 50%, the liquidus temperatureis increased, and the devitrification resistance is degraded. Therefore,the content of La₂O₃ is adjusted to 5 to 50%. The content of La₂O₃ ispreferably in the range of 5 to 45%, more preferably in the range of 5to 40%, still more preferably in the range of 5 to 35%, yet morepreferably in the range of 7 to 30%, further more preferably in therange of 10 to 25%.

When made to be co-present with La₂O₃, Gd₂O₃ works to decrease theliquidus temperature and greatly improve the devitrification resistance.When the content thereof is less than 0.1%, the refractive indexdecreases, the liquidus temperature increases and the devitrificationresistance and the chemical resistance are degraded. On the other hand,when it exceeds 25%, the liquidus temperature is increased, and thedevitrification resistance is degraded. Therefore, the content of Gd₂O₃is adjusted to 0.1 to 25%. The content of Gd₂O₃ is preferably in therange of 0.1 to 20%, more preferably in the range of 0.1 to 18%, stillmore preferably in the range of 0.1 to 15%, yet more preferably in therange of 0.1 to 12%, further more preferably in the range of 0.1 to 10%,still further more preferably in the range of 1 to 10%.

When made to be co-present with La₂O₃, Y₂O₃ also works to decrease theliquidus temperature and greatly improve the devitrification resistance.When the content thereof is less than 0.1%, the refractive indexdecreases, the liquidus temperature increases, and the devitrificationresistance and the chemical durability are degraded. On the other hand,when it exceeds 20%, the liquidus temperature is increased, and thedevitrification resistance is degraded. Therefore, the content of Y₂O₃is adjusted to 0.1 to 20%. The content of Y₂O₃ is preferably in therange of 0.1 to 18%, more preferably in the range of 0.1 to 15%, stillmore preferably in the range of 0.1 to 13%, yet more preferably in therange of 0.1 to 10%, further more preferably in the range of 0.1 to 7%,still further more preferably in the range of 5 to 7%.

When made to be co-present with La₂O₃, Yb₂O₃ also works to decrease theliquidus temperature and greatly improve the devitrification resistance.When the content thereof exceeds 20%, the liquidus temperature isincreased, and the devitrification resistance is degraded. Therefore,the content of Yb₂O₃ is adjusted to 0 to 20%. The content of Yb₂O₃ ispreferably in the range of 0 to 18%, more preferably in the range of 0to 16%, still more preferably in the range of 0 to 14%, yet morepreferably in the range of 0 to 12%, further more preferably in therange of 0 to 10%, still further more preferably in the range of 0 to5%.

ZrO₂ works to increase the refractive index and improve the chemicaldurability. Even when introduced in a small amount, it produces anexcellent effect. However, when the content thereof exceeds 25%, theglass transition temperature and the liquidus temperature increase, andthe devitrification resistance decreases. Therefore, the content of ZrO₂is adjusted to 0 to 25%. The content of ZrO₂ is preferably in the rangeof 0 to 22%, more preferably in the range of 2 to 22%, still morepreferably in the range of 2 to 20%, yet more preferably in the range of2 to 18%, further more preferably in the range of 2 to 15%, stillfurther more preferably in the range of 2 to 13%.

TiO₂ works to increase the refractive index and improve the chemicaldurability and the devitrification resistance. When the content thereofexceeds 25%, however, it is difficult to obtain the desired Abbe'snumber, and moreover, the glass transition temperature and the liquidustemperature are increased, and the devitrification resistance isdegraded. Therefore, the content of TiO₂ is adjusted to 0 to 25%. Thecontent of TiO₂ is preferably in the range of 0 to 22%, more preferablyin the range of 3 to 20%, still more preferably in the range of 3 to18%, yet more preferably in the range of 3 to 17%, further morepreferably in the range of 3 to 16%.

Nb₂O₅ increases the refractive index and also works to decrease theliquidus temperature and improve the devitrification resistance. Whenthe content thereof exceeds 20%, the liquidus temperature is increased,the devitrification resistance is degraded, and it is difficult torealize the desired Abbe's number. Further, the glass is more intenselycolored. Therefore, the content of Nb₂O₅ is adjusted to 0 to 20%. Thecontent of Nb₂O₅ is preferably in the range of 0 to 18%, more preferablyin the range of 0 to 15%, still more preferably in the range of 0 to12%, yet more preferably in the range of 0 to 10%, further morepreferably in the range of 0 to 8%.

Ta₂O₅ works not only to realize the high-refractivity low-dispersionproperties and also to improve the glass stability. Since, however, itis an expensive component, the content thereof is limited to 10% or lessfor achieving the stably supply of a high-refractivity low-dispersionglass that is an object of this invention. Further, when the contentthereof exceeds 10%, the liquidus temperature is increased, and thedevitrification resistance is degraded. Therefore, the content of Ta₂O₅is adjusted to 0 to 10%. The content of Ta₂O₅ is preferably in the rangeof 0 to 7%, more preferably in the range of 0 to 5%, still morepreferably in the range of 0 to 4%, yet more preferably in the range of0 to 3%, further more preferably in the range of 0 to 2%, still furthermore preferably in the range of 0 to 1%. It is particularly preferred toincorporate no Ta₂O₅.

WO₃ is an essential component that increases the refractive index, thatdecreases the liquidus temperature and that serves to improve thedevitrification resistance. When the content thereof is 0.1% or less, itis difficult to obtain the desired refractive index, and further, theliquidus temperature increases, and the devitrification resistance isdegraded. On the other hand, when the content thereof exceeds 20%, theliquidus temperature is increased, and the devitrification resistance isdegraded. Further, the glass is more intensely colored. Therefore, thecontent of WO₃ is adjusted to over 0.1% but not more than 20%. Thecontent of WO₃ is preferably in the range of 0.1 to 18%, more preferablyin the range of 0.1 to 15%, still more preferably in the range of 0.5 to10%, yet more preferably in the range of 0.5 to 8%, further morepreferably in the range of 0.5 to 7%.

GeO₂ is a network-forming oxide and works to increase the refractiveindex, so that it is a component that increases the refractive indexwhile maintaining the glass stability. Since, however, it is a veryexpensive component, it is desirable to decrease its amount togetherwith the Ta component amount. In this invention, the composition isdetermined as already described, so that even if the content of GeO₂ isdecreased to less than 3%, the desired optical properties and excellentglass stability can be together brought to realization. Therefore, thecontent of GeO₂ is adjusted to 0 to less than 3%. The content of GeO₂ ispreferably in the range of 0 to 2%, more preferably in the range of 0 to1%, still more preferably in the range of 0 to 0.5%. Containing no GeO₂or being a Ge-free glass is particularly preferred.

Bi₂O₃ works not only to increase the refractive index but also toimprove the glass stability. However, when the content thereof exceeds10%, the light transmittance in a visible light region is decreased.Therefore, the content of Bi₂O₃ is adjusted to 0 to 10%. The content ofBi₂O₃ is preferably in the range of 0 to 5%, more preferably in therange of 0 to 2%, still more preferably in the range of 0 to 1%. It isparticularly preferred to incorporate no Bi₂O₃.

When introduced in a small amount, Al₂O₃ works to improve the glassstability and the chemical durability. However, when the content thereofexceeds 10%, the liquidus temperature is increased, and thedevitrification resistance is degraded. Therefore, the content of Al₂O₃is adjusted to 0 to 10%. The content of Al₂O₃ is preferably in the rangeof 0 to 5%, more preferably in the range of 0 to 2%, still morepreferably in the range of 0 to 1%. It is preferred to incorporate noAl₂O₃.

Sb₂O₃ can be added as a refiner, and when added in a small amount, itworks to inhibit a decrease in light transmittance which is caused bythe inclusion of impurities such as Fe. However, when it is added in anamount of over 1 mass % based on the glass composition excluding Sb₂O₃,the glass is colored, or it aggravates the deterioration of moldingsurface of a press mold during press-molding due to its strong oxidizingaction. Therefore, the amount of Sb₂O₃ to be added on the basis of theglass composition excluding Sb₂O₃ is preferably 0 to 1 mass %, morepreferably 0 to 0.5 mass %.

SnO₂ can be also added as a refiner. However, when it is added in anamount of over 1 mass % based on the glass composition excluding SnO₂,the glass is colored, or it aggravates the deterioration of moldingsurface of a press mold during press-molding due to its oxidizingaction. Therefore, the amount of SnO₂ to be added on the basis of theglass composition excluding SnO₂ is preferably 0 to 1 mass %, morepreferably 0 to 0.5 mass %.

The optical glass of this invention realizes the optical properties ofhigh-refractivity low-dispersion while maintaining the glass stability,and it hence obviates the incorporation of components such as Lu and Hf.Since Lu and Hf are expensive components, it is preferred to limit thecontent of each of Lu₂O₃ and HfO₂ to 0 to 1%, and it is more preferredto limit the content of each to 0 to 0.5%. Introducing none of Lu₂O₃ andHfO₂ is particularly preferred.

It is also preferred to introduce none of As, Pb, U, Th, Te and Cd bytaking account of adversary effects on the environment.

For making the best use of the excellent light transmittance of theglass, it is preferred to introduce none of substances that cause theglass to be colored, such as Cu, Cr, V, Fe, Ni, Co, etc.

(Properties of Optical Glass)

The refractive index nd of the optical glass of this invention is 1.86to 1.95. The lower limit of the refractive index nd is preferably 1.87,more preferably 1.88, still more preferably 1.89, and the upper limitthereof is preferably 1.94, more preferably 1.93, still more preferably1.92.

A glass having a small Abbe's number vd, i.e., a glass having highdispersion makes it easier to increase the refractive index whilemaintaining the stability. In this invention, therefore, the lower limitof the Abbe's number vd is defined relative to the refractive index nd.The Abbe's number vd of the optical glass of this invention is(2/36−nd)/0.014 or more but less than 38. The lower limit of the Abbe'snumber vd is preferably (2.356−nd)/0.0137, more preferably(2.356−nd)/0.0187.

The optical glass of this invention is a glass suitable for forming aflat and smooth optical-function surface by cutting and polishing. Thesuitability of cold processings such as cutting, polishing, etc., i.e.,cold processability is related to the glass transition temperaturealthough it is indirect. A glass having a low glass transitiontemperature is more suitable to precision press-molding than to coldprocessing, whereas a glass having a high glass transition temperatureis more suitable to cold processing than to precision press-molding, andit has excellent cold processability. In this invention, therefore, whencold processability comes before anything else, it is preferred not todecrease the glass transition temperature to excess. The glasstransition temperature is preferably adjusted to 630° C. or higher, morepreferably adjusted to 640° C. or higher, and still more preferablyadjusted to 660° C. or higher. However, when the glass transitiontemperature is too high, the heating temperature is high when the glassis re-heated to soften it, a mold used for molding is intenselydeteriorated, the annealing temperature is high, and the annealingfurnace is intensely deteriorated or worn out. Therefore, the glasstransition temperature is preferably adjusted to 720° C. or lower, morepreferably adjusted to 710° C. or lower, still more preferably adjustedto less than 700° C.

(Process for Producing Optical Glass)

The process for the production of an optical glass, provided by thisinvention, will be explained below. For example, powdered compound rawmaterials or cullet raw materials are weighed and formulated so as tocorrespond to an intended glass composition, and the formulated rawmaterials are supplied into a melting vessel and then melted by heating.The above raw materials are completely melted to form a glass, and then,this molten glass is temperature-increased to refine it. The refinedmolten glass is homogenized by stirring it with a stirrer, and therefined glass is continuously supplied into a glass flow pipe and causedto flow out, followed by rapid cooling and solidification, to obtain aglass shaped material.

The press-molding glass gob of this invention will be explained below.

[Press-Molding Glass Gob]

The press-molding glass gob of this invention is characteristicallyformed of the above optical glass of this invention. The gob is shapedinto an easily press-moldable form depending upon the form of anintended press-molded product. Further, the mass of the gob isdetermined so as to be suited to a press-molded product. In thisinvention, the glass used has excellent stability, so that the glassdoes not easily devitrify even when it is press-molded by re-heating andsoftening, and shaped products of high quality can be stably produced.

Examples of production of the press-molding glass gob are as follows.

In a first production example, a molten glass flowing out of a pipe iscontinuously cast into a mold that is horizontally placed below the flowpipe, and it is shaped in the form of a plate having a constantthickness. The shaped glass is continuously withdrawn in the horizontaldirection from an opening portion provided on a side surface of themold. The withdrawal of the shaped material in the form of a plate iscarried out by means of a belt conveyor. The withdrawal speed of thebelt conveyor is set at a constant speed, and the glass shaped materialis withdrawn such that it has a constant thickness, whereby a glassshaped material having a predetermined thickness and a predeterminedwidth can be obtained. The glass shaped material is carried to anannealing furnace by means of a belt conveyor and gradually cooled. Thegradually cooled glass shaped material is cut or split in the platethickness direction, and each cut piece is polished or barrel-polishedto form press-molding glass gobs.

In a second production example, a molten glass is cast into acylindrical mold in place of the above mold, to shape a columnar glassshaped material. The glass shaped material obtained in the mold iswithdrawn vertically downward from an opening portion of bottom of themold at a constant speed. The withdrawal speed can be determined suchthat the liquid level of the molten glass in the mold is constant. Theglass shaped material is gradually cooled and then, it is cut or split,and each cut piece is polished or barrel-polished to form press-moldingglass gobs.

In a third production example, a shaping machine having a plurality ofshaping molds arranged on a circular turn table at equal intervals isplaced below the flow pipe, the turn table is index-turned, one of stoppositions of the shaping molds is employed as a position where moltenglass is supplied to shaping mold (to be referred to as “castingposition”), molten glass is supplied to shaping mold on the castingposition, the supplied molten glass is shaped into a glass shapedmaterial, and then the glass shaped material is taken out at apredetermined shaping mold stop position (take-out position) differentfrom the casting position. Which stop position is to be employed as thetake-out position can be determined by taking account of the turn speedof the turn table, the cooling speed of the glass, etc. The supply ofmolten glass to the shaping mold in the casting position can be carriedout by a method in which molten glass is dropped from the flow outlet ofthe flow pipe and the glass drop is received with the above shapingmold, a method in which the shaping mold staying in the casting positionis brought near to the glass flow outlet to support the lower endportion of the molten glass flow, a narrow portion is formed in themiddle of the glass flow, the shaping mold is rapidly moved verticallydownward timely as is predetermined thereby to separate molten glasslower than the narrow portion and the separated molten glass is receivedon the shaping mold, or a method in which the molten glass flow that isflowing out is cut with a cutting blade and the thus-separated moltenglass gob is received with the shaping mold staying in the castingposition.

For shaping a glass on the shaping mold, a known method can be used. Inparticular, when a glass is shaped while the glass is floated byejecting a gas upward from the shaping mold and applying a gas pressureto a glass gob upwardly, the formation of creases on the glass shapedmaterial surface and the cracking of the glass shaped material bycontact to the shaping mold can be prevented.

The form of the glass shaped material may be a form of a sphere, a formof a spheroid, a form having one axis of rotational symmetry and havingtwo planes facing the axis of rotational symmetry and being convexoutwardly, etc. These forms are suitable for glass gobs that arepress-molded to produce optical elements such as a lens, etc., oroptical element blanks. The thus-obtained glass shaped material can beused as a press-molding glass gob as it is, or its surface is polishedor barrel-polished to give a press-molding glass gob.

[Optical Element]

The optical element of this invention will be explained below.

The optical element of this invention is characteristically formed ofthe above optical glass of this invention. The optical element of thisinvention has high-refractivity low-dispersion properties, and expensivecomponents such as Ta₂O₅, GeO₂, etc., are controlled such that theircontents are very small or zero, so that optical elements such asoptically valuable various lenses, prisms, etc., can be provided at alow cost.

Examples of the lenses include various lenses whose lens surface each isspherical or aspherical such as a concave meniscus lens, a convexmeniscus lens, a biconvex lens, a biconcave lens, a plano-convex lens, aplano-concave lens, etc.

These lenses are suitable as lenses for correcting chromatic aberrationsince they can correct chromatic aberration when combined with a lensformed of a high-refractivity high-dispersion glass. Further, they arelenses effective for downsizing an optical system.

Further, the prisms have a high refractive index, and therefore, whenthey are incorporated into an image-sensing optical system to bend alight path toward a desired direction, there can be realized a compactoptical system of wide angle of view.

The optical-function surface of the optical element of this inventionmay be provided with a film that controls a light transmittance, such asan anti-reflection film, etc.

[Process for Producing an Optical Element Blank]

The process for the production of an optical element blank, provided bythis invention, will be explained below.

The process for the production of an optical element blank, provided bythis invention, includes the following two embodiments.

(First Process for Producing an Optical Element Blank)

The first process for the production of an optical element blank,provided by this invention, is a process for the production of anoptical element blank that is to be cut and polished to complete anoptical element, in which the above press-molding glass gob of thisinvention is softened under heat and press-molded.

The optical element blank is a glass shaped material having a form thatis obtained by adding a working margin to be removed by cutting andgrinding to the form of an intended optical element and that is similarto the form of an optical element.

For producing the optical element blank, there is provided a press moldhaving a molding surface having a reversed form of the form of saidblank. The press mold is constituted of mold parts including an uppermold member, a lower mold member and optionally a sleeve member, and themold surfaces of the upper and lower mold members are formed in theabove form, or when the sleeve member is used, the molding surface ofthe sleeve member is formed in the above form.

Then, a mold release agent in the form of a powder, such as boronnitride, or the like is uniformly applied to the press-molding glassgob, the glass gob is softened under heat and then introduced onto thelower mold member, and the softened glass gob is pressed with the uppermold member facing the lower mold member to shape an optical elementblank.

Then, the optical element blank is released from the press mold, takenout of the press mold and then annealed. By this annealing treatment, astrain inside the glass is decreased to bring optical properties such asa refractive index, etc., into desired values.

The condition for heating the glass gob, the press-molding condition andthe material used for the press mold can be selected from knownconditions and materials. The above steps can be carried out inatmosphere.

(Second Process for Producing an Optical Element Blank)

The second process for the production of an optical element blank,provided by this invention, is a process for the production of anoptical element blank that is to be cut and polished to complete anoptical element, in which glass raw materials are melted and theresultant molten glass is press-molded to produce an optical elementblank formed of the above optical glass of this invention.

A press mold is constituted of mold parts including an upper moldmember, a lower mold member and optionally a sleeve member. The moldingsurface of the press mold is processed in the reversed form of thesurface form of an optical element blank as described above.

A mold release agent in the form of a powder such as boron nitride, orthe like is uniformly applied onto the molding surface of the lower moldmember, a molten glass obtained by melting according to the aboveprocess for the production of an optical glass is caused to flow outonto the molding surface of the lower mold member, and when the amountof molten glass on the lower mold member reaches a desired amount, themolten glass flow is cut off with cutting blade(s) called shears. Afterthe molten glass gob is obtained on the lower mold member, the lowermold member with the molten glass gob is moved to a position where theupper mold member stands by above, and the glass is pressed with theupper mold member and the lower mold member, to shape an optical elementblank.

Then, the optical element blank is released from the press mold, takenout of the press mold and annealed. By this annealing treatment, astrain inside the glass is decreased to bring optical properties such asa refractive index, etc., into desired values.

The condition for heating the glass gob, the press-molding condition andthe material used for the press mold can be selected from knownconditions and materials. The above steps can be carried out inatmosphere.

The process for the production of an optical element, provided by thisinvention, will be explained below.

[Process for Producing an Optical Element]

The process for the production of an optical element, provided by thisinvention, comprises cutting and polishing the optical element blankproduced by each of the above processes of this invention. The cuttingand polishing can be carried out by applying known methods.

EXAMPLES

This invention will be explained with reference to Examples hereinafter,while this invention shall not be limited by these Examples.

Example 1

Carbonates, nitrates, hydroxides, oxides, boric acid, etc., were used asraw materials, and powders of the raw materials were weighed and fullymixed to obtain formulated raw materials for obtaining glasses Nos. 1 to10 having compositions shown in Table 1. The formulated raw materialswere placed in platinum crucibles and heated at 1,400° C. to melt them,and the molten glasses were refined and stirred to obtain homogeneousmolten glasses. These molten glasses were cast into pre-heated molds andrapidly cooled, and they were maintained at temperatures around theirglass transition temperatures for 2 hours and then gradually cooled togive optical glasses named glass Nos. 1 to 10. No crystal precipitationwas found in any of the glasses.

The glasses were measured for properties by methods shown below. Table 2shows the measurement results.

(1) Refractive Index nd and Abbe's Number vd

An optical glass obtained by gradual cooling at a rate of 30° C/hour wasmeasured.

(2) Glass Transition Temperature Tg

Measured with a thermomechanical analyzer under the conditions of atemperature elevation rate of 4° C./minute.

(3) Liquidus Temperature LT

A glass was placed in a furnace heated to a predetermined temperatureand held there for 2 hours. After the glass was cooled, an inside of theglass was observed through an optical microscope with a magnification of100 times, and a liquidus temperature was determined on the basis ofwhether or not a crystal existed.

(4) Viscosity at Liquidus Temperature

A viscosity was measured by a viscosity measurement method using acoaxial double cylinder type rotational viscometer according toViscosity JIS Standard Z8803.

(5) Specific Gravity

Measured by an Archimedean method.

TABLE 1 Glass No. 1 2 3 4 5 6 7 8 9 10 SiO2 Mol % 17.0 16.8 16.6 19.320.8 20.8 20.9 20.8 20.5 20.5 Mass % 6.9 6.9 6.9 8.1 8.8 8.7 8.8 8.7 8.99.0 B2O3 Mol % 25.8 25.5 23.8 21.9 21.8 21.8 21.9 21.9 21.5 21.4 Mass %12.1 12.0 11.5 10.7 10.7 10.6 10.6 10.6 10.9 10.7 ZnO Mol % 10.0 10.014.3 12.6 9.7 9.7 9.7 9.7 11.0 11.0 Mass % 5.5 5.5 8.1 7.2 5.5 5.5 5.55.5 6.5 6.5 La2O3 Mol % 19.3 19.1 18.8 18.6 18.6 19.3 18.6 18.6 16.818.3 Mass % 42.3 42.3 42.5 42.3 42.4 44.0 42.4 42.2 39.7 43.3 Gd2O3 Mol% 4.2 4.2 4.1 4.1 4.1 4.1 4.1 4.1 4.0 2.6 Mass % 10.4 10.4 10.4 10.410.4 10.3 10.4 10.3 10.6 6.8 Y2O3 Mol % 2.3 2.3 2.3 2.2 2.2 1.5 2.2 2.22.2 2.2 Mass % 3.5 3.5 3.5 3.5 3.5 2.3 3.5 3.5 3.6 3.6 ZrO2 Mol % 7.37.2 7.1 7.0 7.0 7.0 6.3 6.2 6.9 6.9 Mass % 6.0 6.0 6.1 6.0 6.0 6.0 5.45.4 6.1 6.2 TiO2 Mol % 10.0 10.0 9.8 11.1 12.6 12.6 12.7 12.6 13.9 13.9Mass % 5.4 5.4 5.4 6.2 7.1 7.0 7.1 7.0 8.0 8.1 Nb2O5 Mol % 1.8 1.8 1.71.7 1.7 1.7 2.1 1.7 1.7 1.7 Mass % 3.2 3.2 3.2 3.2 3.2 3.2 3.9 3.2 3.23.3 Ta2O5 Mol % 0.8 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Mass % 2.3 0.00.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 WO3 Mol % 1.5 3.1 1.5 1.5 1.5 1.5 1.52.2 1.5 1.5 Mass % 2.4 4.8 2.4 2.4 2.4 2.4 2.4 3.6 2.5 2.5 Total Mol %100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 Mass % 100.0100.0 100.0 100.0 100.1 100.1 100.1 100.1 100.1 100.1 Sb2O3 Mol % — — —— — — — — — — Mass % 0 0 0 0 0.1 0.1 0.1 0.1 0.1 0.1 SiO2/B2O3 Mol % — —— — — — — — — — Mass % 0.57 0.58 0.6 0.76 0.82 0.82 0.83 0.82 0.82 0.84(Note 1) Amounts of Sb₂O₃ are based on glass composition excludingSb₂O₃. (Note 2) SiO₂/B₂O₃ means a value obtained by dividing content ofSiO₂ by mol % with content of B₂O₃ by mol %. (Note 3) Total (mass %)means a value obtained by adding amount of Sb₂O₃.

TABLE 2 Viscosity Liquidus at liquidus Glass (2.36 − nd)/ Tg temperaturetemperature Specific No. nd νd 0.014 (° C.) (° C.) (dPa · s) gravity 11.89879 36.27 32.94 681 1210 3 5.07 2 1.89919 35.9 32.92 677 1210 3 5.053 1.89985 36.31 32.87 675 1200 — 5.05 4 1.90218 35.73 32.70 680 1210 —5.03 5 1.90315 35.24 32.63 688 1220 2.32 4.98 6 1.90524 35.18 32.48 6881230 — 5.01 7 1.90472 34.97 32.52 685 1220 — 5.01 8 1.90561 34.86 32.46688 1200 — 4.99 9 1.90446 34.4 32.54 683 1200 — 4.93 10 1.90496 34.4232.50 679 1200 — 4.93

Comparative Example 1

Raw materials were prepared so as to obtain a composition thatmaintained content ratios of components other than Ta₂O₅ but had a Ta₂O₅content of zero in the composition No. 37 in Table 8 of JP 2007-269584A.These raw materials were melted under heat and a melt was cast into amold and rapidly cooled. As a result, the entire glass devitrified toturn opaque as shown in FIG. 1, left.

Comparative Example 2

Raw materials were prepared so as to obtain a composition that was thesame as the composition No. 37 in Table 8 of JP 2007-269584A except thatthe entire content of Ta₂O₅ was replaced with La₂O₃, Gd₂O₃, TiO₂, Nb₂O₅,WO₃ and ZrO₂ in equal proportions which were other components forimparting a high refractive index. The raw materials were melted underheat and a melt was cast into a mold and rapidly cooled. As a result,the entire glass devitrified to turn opaque as shown in FIG. 1, right.

Example 2

Press-molding glass gobs formed of the optical glasses Nos. 1 to 10 inExample 1 were produced in the following manner.

First, glass raw materials were prepared so as to obtain the aboveglasses, and they were placed in platinum crucibles, melted under heat,refined and stirred to give homogeneous molten glasses. Each moltenglass was caused to flow out of a flow pipe at a constant flow rate andcast into a mold that was horizontally arranged below the flow pipe toform glass plates having a constant thickness. Each of the thus-formedglass plates was continuously withdrawn in the horizontal directionthrough an opening portion provided on a side surface of the mold,carried into an annealing furnace with a belt conveyor and graduallycooled.

The gradually cooled glass plates were cut or split to make glass piecesand these glass pieces were barrel-polished to obtain press-moldingglass gobs.

In addition, press-molding glass gobs can be also obtained by arranginga cylindrical mold below the flow pipe, casting the molten glass intothe mold to shape columnar glass, withdrawing columnar glass verticallydownward through an opening portion of the mold bottom at a constantspeed, then gradually cooling the glass, cutting or splitting the glassto make glass pieces and barrel-polishing the glass pieces.

Example 3

A molten glass was caused to flow out of a flow pipe in the same manneras in Example 2, and a lower end of the molten glass that was flowingout was received with a shaping mold. Then, the shaping mold was rapidlymoved downward to cut the molten glass flow on the basis of a surfacetension, to obtain a molten glass mass having a predetermined weight onthe shaping mold. And, gas was ejected from the shaping mold to apply agas pressure upwardly to the glass, the glass was shaped into a glassmass while it was floated, and the glass mass was taken out of theshaping mold and annealed. The glass mass was barrel-polished. In thismanner, press-molding glass gobs formed of the same glasses as those inExample 2 were obtained.

Example 4

A mold release agent that was a boron nitride powder was uniformlyapplied to the entire surface of each of the press-molding glass gobsobtained in Example 3, and the above glass gobs were softened under heatand press-molded to produce blanks of various lenses such as concavemeniscus lenses, convex meniscus lenses, biconvex lenses, biconcavelenses, plano-convex lenses, plano-concave lenses, etc, and prisms,

Example 5

Molten glasses were prepared in the same manner as in Example 2, andeach molten glass was supplied onto the molding surface of a lower moldmember to which a mold release agent that was a boron nitride powder hadbeen applied. When the amount of each molten glass on the lower moldmember reached a predetermined amount, the molten glass flow was cutwith cutting blade(s).

Each of the thus-obtained molten glass gobs on the lower mold memberswas pressed with the upper mold member and the lower mold member, toproduce blanks of various lenses such as concave meniscus lenses, convexmeniscus lenses, biconvex lenses, biconcave lenses, plano-convex lenses,plano-concave lenses, etc., and prisms.

Example 6

The blanks obtained in Examples 4 and 5 were annealed. The annealing iscarried out to decrease a strain inside each glass and bring opticalproperties such as a refractive index, etc., into desired values.

The blanks were ground and polished to produce various lenses such asconcave meniscus lenses, convex meniscus lenses, biconvex lenses,biconcave lenses, plano-convex lenses, plano-concave lenses, etc., andprisms. An anti-reflection film may be coated on the surface of each ofthe thus-obtained optical elements.

Example 7

Glass plates and columnar glasses were prepared in the same manner as inExample 2, and the thus-obtained glass shaped materials were annealed todecrease a strain inside each glass and bring optical properties such asa refractive index, etc., into desired values.

Then, the glass shaped materials were cut, ground and polished toproduce blanks of various lenses such as concave meniscus lenses, convexmeniscus lenses, biconvex lenses, biconcave lenses, plano-convex lenses,plano-concave lenses, etc., and prisms. An anti-reflection film may becoated on the surface of each of the thus-obtained optical elements.

INDUSTRIAL UTILITY

This invention is an optical glass that can be stably supplied and thathas excellent glass stability and high-refractivity low-dispersionproperties, and is suitable for press-molding glass gobs, opticalelement blanks and optical elements.

1.-8. (canceled)
 9. An optical glass comprising, by mol %, 0.1 to 40% ofSiO₂, 10 to 50% of B₂O₃, 0 to 10% of total of Li₂O, Na₂O and K₂O, 0 to10% of total of MgO, CaO, SrO and BaO, 0.5 to 22% of ZnO, 5 to 50% ofLa₂O₃, 0.1 to 25% of Gd₂O₃, 0.1 to 20% of Y₂O₃, 0 to 20% of Yb2O3, 0 to25% of ZrO₂, 0 to 25% of TiO₂, 0 to 20% of Nb₂O₅, 0 to 7% of Ta₂O₅, over0.1% but not more than 20% of WO₃, 0 to less than 3% of GeO₂, 0 to 10%of Bi₂O₃, and 0 to 10% of Al₂O₃, the mass ratio of the content of SiO₂to the content of B₂O₃, SiO₂/B₂O₃, being 1 or less, the optical glasshaving a refractive index nd of 1.86 to 1.95, an Abbe's number vd of(2.36−nd)/0.014 or more but less than 38, and a glass transitiontemperature of equal to or greater than 640° C.
 10. The optical glass ofclaim 9, which has a refractive index nd of 1.89 to 1.95, an Abbe'snumber vd of (2.36−nd)/0.014 or more but less than
 38. 11. The opticalglass of claim 9, wherein the content of ZnO is 0.5 to 18 mol %.
 12. Theoptical glass of claim 9, which is a Ge-free glass.
 13. The opticalglass of claim 9, wherein the content of SiO₂ is 3 to 35 mol %, and thecontent of B₂O₃ is 12 to 45 mol %.
 14. The optical glass of claim 9,wherein the mass ratio of the content of SiO₂ to the content of B₂O₃,SiO₂/B₂O₃, being 0.95 or less.
 15. The optical glass of claim 9, whereinthe total content of Li₂O, Na₂O and K₂O is 0 to 8 mol %.
 16. The opticalglass of claim 9, wherein the total content of MgO, CaO, SrO and BaO is0 to 8 mol %.
 17. The optical glass of claim 9, comprising, by mol %, 5to 45% of La₂O₃, 0.1 to 20% of Gd₂O₃, 0.1 to 18% of Y₂O₃, and 0 to 18%of Yb₂O₃.
 18. The optical glass of claim 9, wherein the content of ZrO₂is 0 to 22 mol %.
 19. The optical glass of claim 9, wherein the contentof TiO₂ is 0 to 22 mol %.
 20. The optical glass of claim 9, wherein thecontent of Nb₂O₅ is 0 to 18 mol %.
 21. The optical glass of claim 9,wherein the content of Ta₂O₅ is 0 to 5 mol %.
 22. The optical glass ofclaim 9, wherein the content of WO₃ is greater than 0.1 mol % but equalto or less than 18 mol %.
 23. The optical glass of claim 9, wherein thecontent of Bi₂O₃ is 0 to 5 mol %.
 24. The optical glass of claim 9,wherein the content of Al₂O₃ is 0 to 5 mol %.
 25. The optical glass ofclaim 9, wherein the content of Sb₂O₃ is 0 to 1 mass % based on theglass composition excluding Sb₂O₃.
 26. The optical glass of claim 9,which has a glass transition temperature of equal to or greater than660° C.
 27. The optical glass of claim 9, which has a glass transitiontemperature of equal to or less than 720° C.
 28. The optical glass ofclaim 9, which has a specific gravity of 4.93 to 5.07.